ΔE 12.1, fair: the same colour family, but not a straight swap. Dark Gull Gray is noticeably lighter than N4 Neutral Gray.
Can I use Dark Gull Gray instead of N4 Neutral Gray?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Dark Gull Gray is noticeably lighter than N4 Neutral Gray.
Can I use N4 Neutral Gray instead of Dark Gull Gray?
The same difference applies the other way round: N4 Neutral Gray is noticeably darker.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.